Known in the art is a diesel engine arranging a control valve at the top face of each combustion chamber in addition to an intake valve and exhaust valve and provided with a control means for opening this control valve at the time of start of the compression stroke and closing it in the middle of the compression stroke (see Japanese Patent Publication (A) No. 4-86338). In this diesel engine, even if the compression stroke is started, while the control valve is open, the intake air in the combustion chamber is exhausted through the control valve, so no compression action is performed. The compression action is started when the control valve closes. Therefore, in this diesel engine, the closing timing of the control valve is controlled to control the compression ratio.
Note that, in this diesel engine, by controlling the closing timing of the control valve, the lower the atmospheric pressure, the higher the compression ratio is made, while the lower the atmospheric temperature, the higher the compression ratio is made.
On the other hand, in a spark ignition type internal combustion engine, the output torque of the engine is determined by the amount of intake air. In this case, by controlling the closing timing of the intake valve, it is possible to control the amount of intake air in the control combustion chamber. That is, even if the compression stroke has started, while the intake valve is open, the intake air in the combustion chamber is exhausted through the intake valve into the intake port, so the amount of intake air actually fed into the combustion chamber becomes the amount of intake air sealed in the combustion chamber when the intake valve closes. Therefore, by controlling the closing timing of the intake valve, the amount of intake air of the combustion chamber can be controlled.
In this regard, in an internal combustion engine, the output torque required in accordance with the operating state of the engine is preferably generated even if the atmospheric pressure changes. For this reason, the mass of the intake air fed into the combustion chamber has to be maintained the same even if the atmospheric pressure changes. Therefore, for example, if the atmospheric pressure falls, the density of the intake air will become lower, so the volume of the intake air fed into the combustion chamber will have to be increased. Therefore, when trying to close the intake valve after intake bottom dead center, at this time it is necessary to advance the closing timing of the intake valve.
In this regard, if advancing the closing timing of the intake valve, the compression ratio will become higher, so if for example assuming the atmospheric temperature is the same, the compression end temperature will end up becoming extremely high. On the other hand, in this case, to lower the compression end temperature, the closing timing of the intake valve may be delayed, but if delaying the closing timing of the intake valve, this time the amount of intake air will be reduced and consequently the output torque will end up becoming lower than the required torque.
In the above-mentioned known diesel engine, by controlling the closing timing of the control valve, the compression ratio is controlled to the target compression ratio in accordance with the atmospheric pressure and atmospheric temperature. That is, the compression end pressure and compression end temperature are controlled to the compression end pressure and compression end temperature suited for compression ignition combustion. However, when controlling the closing timing of the intake valve to control the amount of intake air fed to the combustion chamber, even if the compression end temperature becomes extremely high, the closing timing of the intake valve cannot be delayed to obtain an output torque in accordance with the required torque. To control the compression end temperature, still other control different from that of the above-mentioned diesel engine becomes required.